Method of die-expressing an aluminum-base alloy



3,177,573 METHOD OF DIE-EXPRESSING ALUMINUM-BASE ALLOY George S. Foerster, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. 810,258. Ser. No.

Original application May 1, 1959, Ser. No. Divided and this application Feb. 24, 1961, 91,311

4 Claims. (Cl. 29420) The invention relates to die-expressed articles of aluminum-base alloy. It more particularly concerns an improved method of producing die-expressed articles of aluminum-base alloys of the dispersion hardened type whereby high values of their mechanical properties are attained which are not appreciably lowered by exposure to, elevated temperatures.

This application is a divisional application of copending application Serial No. 810,258, filed May 1, 1959, now abandoned.

Heretofore high strength aluminum-base alloys have been prepared by alloying aluminum with one or more suitable constituents which are capable of forming solid solutions with aluminum. Such alloys are often further treated, as by solution and precipitation heat treating and cold-Working, to increase yield and tensile strength. However, at elevated temperatures the eifectiveness of solid solution strengthening and precipitation hardening is limited and further the benefits of cold-Working and aging are soon lost because recovery, recrystallization and overaging take place upon heating the alloys to even rather moderate temperatures. In other attempts to produce high strength aluminum-base alloys, fine aluminum powder having a thin oxide coating has ben die-expressed to produce an extrude in which aluminum oxide is Widely and finely dispersed in an aluminum metal matrix. While so-formed extrudes exhibit good mechanical properties, they tend to be quite brittle and diilicult to form even at elevated temperatures. In addition extremely fine aluminum powder is required to provide for sufficiently wide dispersion of the oxide in the extrude as the oxide is present only as a surface coating on the so-particulated metal particles. The preparation of aluminum powder of sufficient fineness is not only expensive but the fine powder is hazardous to handle.

It is therefore an object of the invention to provide an improved method of forming an aluminum-base alloy having superior physical and mechanical properties 'at elevated temperatures.

A further object is to provide an improved method of preparing an aluminum-base alloy which has superior mechanical and physical properties but is still readily formable into useful shapes.

v United States Patent F ,l77 ,57.'i Patented Apr. 13, 1965 A still further object of the invention is to provide an' cal and physical properties which are not appreciably decreased by exposure to elevated temperatures.

These and other objects and advantages of the invention will be more fully understood on becoming familiar with the following description and the appended claims. The invention is predicated on the discovery that by preparing an aluminum-base alloy containing aluminum and one or more constituents which are each miscible with aluminum in the molten state but substantially insoluble in solidified aluminum or which form an intermetallic compound with aluminum which is similarly insoluble in solidified aluminum and by forming said alloy into a mass of atomized particles, heating the mass, compacting the heated mass and then die-expressing the heated compacted mass to form an extrude or die-expressed article, the so-obtained article exhibits exceptionally desirable mechanical properties. F or purposes of the specification and claims the term solidified aluminum is extend-' ed in meaning to include solidified aluminum-base alloy.

- In carrying out the invention, an aluminum-base alloy containing at least 70 percent of aluminum is prepared by alloying, according to well known methods, aluminum and one or more constituent metals having specific properties. An essential constituent metal must have sufficient solubility in molten aluminum at reasonable alloying temperatures, for example 650 to 900 C., to avoid the necessity of employing high temperatures at Whichalurninum readily attacks container materials but a solid solubility in aluminum and aluminum-base alloy of less than 0.1 Weight percent. The use of metals, such as boron, iron and cobalt which exhibit the requisite low solid solubility in solidified aluminum but which are difficult to alloy in eitective amount in molten aluminum is undesirable in the practice of the invention. As to the solid solubility in aluminum, those elements which are soluble in molten aluminum but which on cooling form an intermetallic compound with aluminum which is similarly insoluble in solidified aluminum may also be employed. A molten alloy comprising aluminum and the constituent metal should exhibit a high solidus temperature, that is, above about 600 C., whereby articles formed of the alloy are usable at elevated tem-peratures. It is further 3 desirable that themolten alloy exhibit a narrow solidification range. Suitable metal constituents that may be used singly or in combination in the practice of the invention include gold, barium, beryllium, rare earth metals, palladium, platinum, antimony, selenium, strontium, tellurium, thorium and unanium, the pertinent properties of which are listed in Table I- Table I Liquid Solldus Tem-1 Intennitalic 0031- MW?! pjerce rl oit Metal Solid solubilit Solubilityin perature of A poun orme e a 0115! uen Constituent in Al, Wt. perceiit A1 at 800 0., Binary, C. With Al to Produce 10% by Wt. percent Volume oi-Intermetallic Compound 3 4 v 20 10 50 8 40 12 10 18 22 15 l l 53 20 30 I 18 20 15 1 Similar to barium.

The alloy to be used is brought to the molten state in any convenient manner in preparation for atomizmg. Temperatures in the order of 25 to 50 centigrade degrees above the melting point of the alloy are desirable although oth r pe ur s. m y be us d t which h a oy s m t e molte s a I pr ferabl t u e the owe pe at re of a mo te tate on s to edu e the degree of hazard involved in handling the molten alloy but also to reduce the amount of heat which is to be removed to permit the molten alloy to return to the solid state. It is highly desirable for reasons hereinafter more fully discussed that the solidification of the atomized alloy take" place quickly in order to minimize aggregation or crystal growth of constituents which are insoluble 1n solidified aluminum.

- The alloy while in the molten state is subjected to a dispersion and chilling operation whereby the metal is obtained in atomized form, that is,-in the formof fine 1ndividually frozen discrete particles. There are various ways in which atomization may be performed and any one of them may be used. A convenient method appears to be directing a jet of an inert cooling gas against an unconfined stream of the molten alloy as described in U.S Patent 2,630,623

For example, a freely falling stream of the molten metal may be broken into droplets and solidified by impinging upon the stream an inert gas such as a hydrocarbon gas (e.g., methane, ethane, propane, butane, etc.), argon, helium, hydrogen, the inert gas having a boiling temperature below the melting point of the molten metal. A wide range of particle sizes, although small, usually results from the atomizing operation. The atomized powder comprises more or less spherical particles for the most part ranging in size from about mesh to smaller than 325 mesh. A preferred range of particle size is from 3 25 mesh to about 140 mesh.

' As a result of the atomizing operation, there is imparted to each particle of the aluminum-base alloy a special heterogenous microstructure essential in achieving the objects of the invention. This structure is characterized by an aluminum metal matrix having uniformly dispersed therethrough a discontinuous phase made up of very fine crystallites of an insoluble phase consisting of the said essential alloying metal or an intermetallic compound thereof with aluminum. This uniformly heterogeneous microstructure is exhibited by all the aluminum-base alloys in atomized form (a frozen dispersion of the melt) containing one or more of the aforesaid alloying metal constituents which are substantially insoluble in solidified aluminum or which form insoluble intermetallic compounds, the insoluble form of the added metal constituent totaling by volume from about 0.5 to percent of the alloy and preferably from 5 to 15 percent. The term atomized metal as used herein and in the appended claims has the conventional metallurgical meaning of chilled solidified droplets of molten metal produced by any of the known atomizing methods. The size of droplets that yield the desired microstructure, that is, a fine state of subdivision of the insoluble phases on being chilled to solidification in the atomizing operation appears to be in the range of from 1 tolOO microns average diameter though other sizes may be used. I However atomized particles greater than about 100 microns diameter exhibit a microstructure in which the insoluble phase is not so minutely and widely dispersed as in the smaller atomized particlesf Extruded articles prepared from such coarse particles accordingly show less of the dispersion hardening effect/ As for extremely fine particles, those particles having an average diameter of less than -1 micron are costly to make and very dangerous to handle on account of the explosive hazard.

The requisite rapid cooling of the aluminum-base alloy from the molten state in'which the alloying .m etal constituent is miscible to the solid state in which the alloying constituent is insoluble in aluminum is readily achieved in the atomization process and the tendency of growth or agglomeration of the insoluble phase into larger crystals is thereby reduced or substantially prevented. On the other hand, the same alloying composition when cast in the form of an ingot is found to have .a grain structure in which the insoluble alloying metal or intermetallic compound thereof has undergone extensive crystal growth whereby there'are but fewnelatiyely large crystals of the insoluble solid phase dispersed throughout the grain structure. This may be attributed to the fact that an ingot cannot readily be cooled as rapidly as atomized particles.

In the next step of the method, the atomized metal is heated, preferably in bulk, in an oven or by contact with a heated metal surface, in preparation for compacting and die-expression. Compacting and die-expression may be carried out in conventional apparatus designed for the extrusion of aluminum-base :alloy. A suitable method and apparatus for carrying out the dieexpression of pelletized light metal, such as aluminum, is described in US. Patent 2,630,623. The temperature to which the metal is heated is within the conventional plastic deformation temperature range for aluminum-base alloys, usually between about 260 C. and 510 C. A preferred range is about 315 to 427 C.

It has been found that the as-atomized aluminum-base alloy may be heated in bulk to the desired temperature merely by placing it in a suitable metal vessel in a heated oven or it is possible to charge the heated container of a die-expressing apparatus with as-atomized metal and proceed with the operation of the apparatus to effect dieexpression with substantially no destruction of the asatomized microstructure of the alloy. Normally it takes but a few seconds for the charge in the die-expression apparatus to reach extrusion temperatures. During this interval of time, the charge of metal reaches the plastic deformation temperature :and is compacted by the pressure exerted upon the metal by the plunger of the apparatus before extrusion commences. The hot compacted metal extrudes from the die as soon as extrusion pressures are reached which pressures are comparable to conventional extrusion pressures.

The amount of reduction in the cross-sectional dimensions of the compact effected by the extrusion or die-expression is subject to wide variations and may be from about 8 to 1 to as much as 2500 to l or more (i.e., from about 85 percent to over 99 percent reduction is crosssectional area) depending upon the strength of the apparatus as is well understood in the art.

The extrude so produced is possessed of a uniformly microheterogeneous grain structure characterized by unusually. high values of strength which are not appreciably decreased upon repeatedly heating the extrude to elevated temperatures, such as 540 C. for 1 hour, yet the extruded material is sufiiciently ductile to be readily formable at elevated temperatures by conventional forming techniques.

'The enhancementof the mechanical properties of aluminum-base alloy by the presence'of the herein described fine insoluble solid phase particles is not entirely understood but it is believed that the dispersion is instrumental in inhibiting slippage along crystal planes thereby inhibiting deformation and increasing mechanical properties.

This belief is strengthened by the discovery that the change in properties of an aluminum-base alloy upon a metal eonstituent forming a solid insoluble phase is more directly related to the volume of insoluble phase formed than to'the percent by weight of alloying metal constituent added.

In accordance with the invention, a quantity of each of the aluminum-base alloys listed in Table II in atomized form was die-expressed, In each case the quantity of atomized material was charged into a cylindrical container 3 inches in internal diameter, the container being at 400 C. The charge had a depth of about 6 inches and was compacted at 400 C. in the container to a compact about 4"inches long. The compact was then dieexpressed at the same temperature at the rate of 5 feet per minute into a strip having a rectangular cross section 1% inches by inch, the reduction in area being about 90:1. The so-obtained die-expressed articles were subjected to physical testing at 24 0., 315 C. and 427 C. The results of the tests were listed in Table II.

metals, palladium, platinum, antimony, selenium, strontium, tellurium, thorium, uranium, and mixtures thereof in an amount suflicient to form with the base metal from about 0.5 to about 20 volume percent of the corresponding aluminum intermetallic compound in the alloy, (b) atomizing the molten mixture so as to convert the same Table II Composition Properties at 24 0. Properties at 316 0. Properties at 427 C. Run N 0. Per- Per- Per- Per- Per- Pen Percent cent cent cent cent TYS CYS TS cent TYS TS cent TYS TS Th Ba MM Al E E MM=Mlsch metal.

Percent E=Percent elongation in 2 inches.

TYS=Tensile yield strength in lbs. per sq. in. 1,000. CYS=Compression yield strength in lbs. per sq. in. l,000. TS=Ultimate tensile strength in lbs. per sq. in. 1,000.

.In another embodiment of the invention, an aluminumbase alloy is prepared by making suitable additions to aluminum of a metal constituent, which forms an insoluble phase therewith as described hereinabove as well as one or more metals which increase the strength of aluminum in a well known conventional manner such as by solution hardening. It has thus been found that the benefits of dispersion hardening may be combined with the benefits of increasing the strength of the matrix about the finely dispersed crystallites of solid insoluble phase. Metals which may be added variously to increase the matrix strength include:

Weight percent Ag 0-10 Ca O0.7 Cr 0 0.8 Cu ()6 Li O 5.5 Mg 0-15 Mn 0-2 Si 0-2 Ti 0-1 Zn O-10 Zr O-0.3

Use of combinations of the above-listed elements in amount in which the metals are incompatible by virtue of mutual insolubility in molten aluminum whereby a precipitate is formed which settles from the melt, or in amounts which otherwise fail to increase matrix strength, is of no advantage in the practice of the invention. While simple binary or ternary combinations of the above-listed matrix strengthening constituents are believed to be compatible, it is within the skill of the metallurgist to check desired alloying combinations for incompatibility such as insolubility in molten aluminum. In general, any conventional aluminum-base alloy system may be employed, such as Al-Mg, Al-Cu, Al-Mn, AlSi, A1Zn, Al-Mg-Zn, or AlSiCu, in combination with one or more of the previously mentioned metals which form a solid insoluble phase in solidified aluminum.

Such composite alloys, that is, those including dispersion hardening, as well as conventional strengthening alloying metals, are atomized, compacted, and extruded as described hereinabove.

What is claimed is:

1. The method of making a high strength die-expressed aluminum base alloy article which comprises, (a) providing a molten mixture consisting of at least 70 percent of an aluminum base metal and a second metal selected from the group consisting of gold, barium, rare earth into a mass of fine individually frozen particles having dispersed therethrough fine crystallites of said intermetallic compound, (0) heating the mass of as-atomized alloy to a temperature within the range of plastic deformation of the alloy, (d) applying pressure upon the heated mass so as to compact the same, (e) ram extruding the resulting compacted mass at a temperature within the aforesaid range, the reduction in cross-sectional area of the compacted mass eflected by the ram extrusion being at least about percent.

2. The method of claim 1 wherein, in addition to the base metal and said second metal, the molten mixture provided in step (a) also includes at least one matrix strengthening alloying constituent in an effective strengthening amount.

3. The method of making a high strength die-expressed aluminum base alloy article which comprises, (a) providing a molten mixture consisting of at least 70 percent of an aluminum base alloy and from about 0.5 to about 20 volume percent of a solid insoluble phase selected from the group consisting of AuAl BaAl CeAl PdAl SbAl, SeAl Sr Al Te Al ThAl UAl and mixtures thereof, (b) atomizing the molten alloy so as to convert the same into a mass of fine individually frozen particles having disposed therethrough fine crystallites of said insoluble phase, (c) heating the mass of as-atomized alloy to a temperature within the range of plastic deformation of the alloy, (d) applying pressure upon the heated mass so as to compact the same, (e) ram extruding the resulting compacted mass at a temperature within the aforesaid range, the reduction in cross-sectional area of the compacted mass effected by the ram extrusion being at least about 80 percent.

4. The method of claim 3 wherein, in addition to the selected solid insoluble phase, the aluminum base alloy of step (a) also includes at least one matrix strengthening alloying constituent in an effective strengthening amount.

References Cited by the Examiner UNITED STATES PATENTS 2,630,623 3/53 Chisolm et a1 29-42O 2,679,932 6/54 Burns 29-420 X 2,749,604 6/56 Latin 29-420.5

OTHER REFERENCES Powder Fabrication of Aluminum Alloys, J. B. Hess and R. S. Mateer, WA D. C. Technical Report 54-590, September 1955.

WHITMORE A. WILTZ, Primary Examiner. NEDWIN BERGER, Examiner. 

1. THE METHOD OF MAKING A HIGH STRENGTH DIE-EXPRESSED ALUMINUM BASE ALLOY ARTICLE WHICH COMPRISES, (A) PROVIDING A MOLTEN MIXTURE CONSISTING OF AT LEAST 70 PERCENT OF AN ALUMINUM BASE METAL AND A SECOND METAL SELECTED FROM THE GROUP CONSISTING OF GLD, BARIUM, RARE EARTH METALS, PALLADIUM, PLATINUM, ANTIMONY, SELENIUM, STRONTIUM, TELLURIUM, THORIUM, URANIUM AND MIXTURES THEREOF IN AN AMOUNT SUFFICIENT TO FORM WITH THE BASE METAL FROM ABOUT 0.5 TO ABOUT 20 VOLUME PERCENT OF THE CORRESPONDING ALUMINUM ILNTERMETALLIC COMPOUND IN THE ALLOY, (B) ATOMIZING THE MOLTEN MIXTURE SO AS TO CONVERT THE SAME INTO A MASS OF FINE INDIVIDUALLY FROZEN PARTICLES HAVING DISPERSED THERETHROUGH FINE CRYSTALLITES OF SAID INTERMETALLIC COMPOUND, (C) HEATING THE MASS OF AS-ATOMIZED ALLOY TO A TEMPERATURE WITHIN THE RANGE OF PLASTIC DEFORMATION OF THE ALLOY, (D) APPLYING PRESSURE UPON THE HEATED MASS SO AS TO COMPACT THE SAME, (E) RAM EXTRUDING THE RESULTING COMPACTED MASS AT A TEMPERATURE WITHIN THE AFORESAID RANGE, THE REDUCTION IN CROSS-SECTIONAL AREA OF THE COMPACTED MASS EFFECTED BY THE RAM EXTRUSION BEING AT LEAST ABOUT 80 PERCENT. 